Amongst those in the paper industry the term "soft paper" is commonly given to a particular grade of paper used for absorbing purposes, such as tissues, drying cloths, paper toweling, napkins and handkerchiefs. These paper products, unlike their woven fabric counterparts, are intended for disposable use. Thus, while both paper and woven products desirably have rapid and effective absorption, soft feel, smooth structure, and good strength in both dry and wet states, it is the peculiar challenge of the paper industry to provide products embodying these characteristics at a price which makes their one-time use cost-effective.
The bulk of soft paper is manufactured by wet-forming. Wet-forming involves the use of a fiber suspension, usually in water, which is placed on a running wire or conveyor belt and subsequently dewatered and dried. High speed machines which acquire speeds of between 500 and 2,000 m/min. are commonly used, and a grammage between about 20 and about 30 g/m.sup.2 is also common. In addition, the wet-formed paper is generally creped, usually by means of a so-called "Yankee cylinder," from which the paper web is scraped off after drying. Creping provides the paper with the necessary extensibility and softness.
Another method of forming soft paper is dry-forming. In dry-forming, dry paper-making pulp is fluffed to form fibers which are suspended in air. The air-borne fibers, without addition of water or other solvent, are deposited on an air pervious wire, and these fibers are bound together by means of a suitable chemical binding agent or agents which are added thereto. Because soft paper manufactured in this way is very bulky, i.e., has a very loose structure, the wire speed of dry-forming apparatus must be significantly reduced relative to the speed of wet-forming machines. Production rates of about 50 m/min. are common. As expected, manufacturing costs are very high because of the low throughput and cost of expensive binders. Consequently, paper manufactured by dry-forming is higher priced and occupies an uncompetitive position in the market place.
This is not to say, however, that price is the only consideration when choosing between soft paper manufactured by wet- and dry-forming. Dry-formed soft paper is a higher bulk than wet-formed paper. This results in a soft and smooth surface more pleasing to the touch. The reason for the higher bulk is that dry-formed paper fibers have not been softened in water and have not been bent down into the plane of the paper, nor have capillary forces been work during the removal of water therefrom. In contrast, wet-formed soft paper is stronger because of the amount of fiber binding which takes place when the fibers are in suspension and as they are dewatered. Furthermore, this strength is obtained without the necessity of additional binding agents which are required by dry-forming soft paper.
Despite significant improvements in the paper-making technology, a paper product having the softness of a dry-formed soft paper and the strength of a wet-formed soft paper, without the required use of expensive binding agents has gone unrealized.
It is therefore an object of the present invention to provide a soft paper product which has the softness of dry-formed paper and the strength and resiliency of wet-formed paper without binders.
It is also an object of the present invention to provide a method for forming a soft paper product having the softness of dry-formed soft paper and the strength of wet-formed soft paper, but without the use of binding agents.
It is further an object of the present invention to provide an apparatus for the production of a soft paper which has the strength of wet-formed soft paper and yet has the softness characteristic of dry-formed soft paper.